[foobar99]

Bidirectional type checking is what makes Swift type checking exponential. Operators and literals just really jack up the numbers in the exponential function because they quickly lead to deeply nested expression trees with many choices on each nesting. So that's why they're the most commonly cited examples. They look very innocent and simple but aren't in Swift.

But you can absolutely construct slow expressions just with functions that are overloaded on both sides (i.e. parameters and return type). Generics and Closures also drive up the complexity a lot, though.